The present invention relates to xe2x80x9cactive implantable medical devicesxe2x80x9d as such devices are defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities, more particularly to pacemaker, defibrillator and/or cardiovertor devices that are able to deliver to the heart stimulation pulses of low energy for the treatment of heartbeat rate disorders. The invention is more particularly directed to the prostheses known as xe2x80x9cmultisitexe2x80x9d, in which respective electrodes are placed in a plurality of distinct respective cardiac sites comprising at least one ventricular site and one atrial site. This prosthesis can be of the xe2x80x9ctriple chamberxe2x80x9d (right atrial stimulation and double ventricular stimulation) or xe2x80x9cquadruple chamberxe2x80x9d (double atrial stimulation and double ventricular stimulation) type.
The control of stimulation implies making an adjustment of various control parameters, such as the stimulation frequency, the atrio-ventricular delay (AVD), or the inter-ventricular delay in the case of a biventricular stimulation. These various parameters are typically adjusted according to signals delivered by sensors, for example, a minute ventilation (MV) sensor. The minute ventilation is a factor which is representative of the instantaneous metabolic needs of the patient. This factor, in a known manner, is evaluated by measurement of a trans-pulmonary bioimpedance, i.e., between the heart and the case of pacemaker, where the case is located in the top of the thorax.
Another factor which is desirable to know is the cardiac flow. It can be interesting, particularly with a multisite pacemaker, to obtain an indication of this flow and thus of the fraction of ejection. The fraction of ejection is the hemodynamic reference parameter used to optimize stimulation on the various cardiac sites. The cardiac flow can be evaluated by measurement of the intracardiac pressure, for example, as proposed in the published application WO-A-99/34863 (Pacesetter AB), but at an expense of requiring a specific probe incorporating a piezoelectric sensor and particular associated electronics to condition the signals resulting from this sensor, to convert them and transmit them to the microprocessor of the pacemaker for processing and use.
Another parameter correlated with the cardiac flow is the transvalvular impedance, a parameter that is generally measured on the right heart, for example, as proposed in U.S. Pat. No. 5,154,171 (Chirife). This document proposes to take the bio-impedance measurement by injecting a current pulse between a ventricular site and an atrial site, and collecting a differential potential between these same two sites. In practice, however, it is noted that this configuration (a bipolar configuration of two electrodes) of injection/collection appears sensitive to the movement of the probes containing the electrodes, and does not allow a reliable and precise measurement of the impedance and of the fraction of ejection.
It is, therefore, an object of the present invention to overcome the foregoing disadvantages by proposing an improved configuration for measuring the intracardiac impedance and procuring a more reliable and more precise measure of the fraction of ejection, in particular for use in controlling the inter-ventricular delay (in the case of a biventricular stimulation), the stimulation frequency, and/or the atrio-ventricular delay.
Broadly, the present invention relates to a medical device of the known general type, for example, a device according to U.S. Pat. No. 5,154,171 mentioned above, i.e., in which electrodes are to be placed in a plurality of distinct respective sites comprising at least one left ventricular site and one right atrial site, or at least one right ventricular site and one left atrial site. These electrodes are to be connected to a collection (detection) circuit, able to collect (detect) cardiac signals including a potential of depolarization, and to a stimulation circuit, able to apply stimulation pulses to at least certain of the aforesaid sites. The stimulation circuit and the collection circuit typically are located inside the case of the device. The device of the present invention also comprises a means for evaluating the cardiac flow by measurement of the intracardiac bio-impedance. It should be understood that the term xe2x80x9csitexe2x80x9d is used synonymously with the term cavity.
In a characteristic manner of the invention, the means for evaluating the cardiac flow further comprises means for measuring a trans-septum bio-impedance, which measurement is made either between the left ventricle and right atrium, or between the right ventricle and the left atrium. The trans-septum bio-impedance measuring means operates by injecting a current between an atrial site and a ventricular site, and collecting a differential potential between an atrial site and a ventricular site.
In a first embodiment, the means for measuring the trans-septum impedance operates the aforementioned current injection between a common ventricular site and a first atrial injection site, and operates the aforementioned collection of the differential potential between the aforementioned common ventricular site and a second atrial collection site.
In another embodiment, the means for measuring the trans-septum bio-impedance operates the aforementioned current injection between a common atrial site and a first ventricular injection site, and operates the aforementioned collection of the differential potential between the aforementioned common atrial site and a second ventricular collection site.
In the foregoing embodiments, in the cavity of the common site, preferably the same electrode is used for the injection and the collection, and in the cavity of the non-common sites, preferably different electrodes in the same cavity are used for the injection and collection, which different electrodes are more preferably disposed on the same lead.
Preferably, the device also includes means for varying the stimulation pulse frequency is applied, means for varying the atrio-ventricular delay applied, and/or means for varying the inter-ventricular delay applied, with respect to the right and left ventricles, all of these means operating in response to the measured trans-septum bio-impedance to vary the parameter in a direction towards an improvement of the cardiac flow.
In addition, the device optionally includes means for detecting ventricular arrhythmias and means for discriminating between, on the one hand, patient effort (i.e., activity above a rest level of activity) accompanied by an elevated heart rate, and, on the other hand, disorders of the heart rate accompanied by a fall of the cardiac flow as detected by the means for measuring the trans-septum bio-impedance.